The most likely cause is a style sheet in the Page Wrapper. If you create a basic Page Wrapper that is clean with no style sheets, you can use this for your events and the popup windows should display correctly. Once you have created a new page wrapper, you can set it for the event here:
  1. Navigate to: Content, Event, event in question
  2. Identify Event (page 1)
  3. Page Wrapper (step 3)
  4. Override the default Page Wrapper
  5. Save